Earthquakes, tsunamis, and the Richter scale

Earthquake in Peru – Earthquakes can split the ground open! What causes earthquakes?

What causes earthquakes?

Earthquakes happen when the moving tectonic plates that make up the surface of the Earth move apart or bump into each other, or slide under each other. This movement tears apart the surface of the Earth, or crunches it up. Most often, this just means a little shaking for a few seconds, and nothing very serious happens.

Several times a year, though, somewhere in the world there is enough movement to really shake the earth a lot, and the earthquake is serious enough to knock down buildings. When the buildings fall on people, many people can be killed in a few minutes. The strongest earthquakes can break trees in half.

What is the Richter scale?

The Richter scale (or ML scale) rates earthquakes on an exponential scale, so that if an earthquake is rated 1, you can hardly feel it, but an earthquake rated 2 is ten times as strong as an earthquake rated 1, and an earthquake rated 3 is ten times as strong as an earthquake rated 2.

How strong was that earthquake?

Only a few people feel a level 1 earthquake. In a level 2 earthquake, a few people who are resting may feel it, especially if they’re near the top of a tall building. Nearly everyone will feel a level 5 earthquake, and some dishes and windows will break. At level 6, heavy furniture moves around, and many people will feel frightened, but there’s not really much damage. In a level 8 earthquake, many buildings will fall down.

A 6.8 earthquake in 2001 in Geiyo, Japan

Earthquakes and tsunamis

Because most of the Earth is covered by oceans, earthquakes often happen in the ocean. Usually an earthquake in the ocean just shakes the water and people don’t notice. But sometimes the water pulls all together into a huge wave called a tsunami (tsoo-NAMM-ee).

Are there earthquakes on other planets?

Predicting earthquakes

People have tried to predict and prevent earthquakes for thousands of years, but we still can’t really predict them, and we can’t do anything to prevent them. Early people made sacrifices to their gods at places like Delphi, where there are a lot of earthquakes, to try to keep the gods happy so they wouldn’t shake the earth.

By the time of Aristotle (300s BC) scientists understood that this was a natural thing the earth just did. The Chinese scientist Zhang Heng built a seismograph in the 100s AD that served as a sort of early warning system. Our seismographs today use pretty much the same idea. By the 800s AD, Ibn Sina in Iran knew that earthquakes shape the geology of a region, thrusting some rocks up and pulling others down.

History of earthquakes

People sometimes think an earthquake can destroy a civilization. It’s true that some earthquakes kill tens or even hundreds of thousands of people, but the survivors usually rebuild their city pretty soon. One early earthquake knocked down the palaces of Crete about 1700 BC, but people rebuilt bigger and better palaces.

A big tsunami hit Peru about 500 BC, but the Chavin also rebuilt. But if a city is already getting smaller, a big earthquake encourages people to abandon it. That’s what happened in Rome, when a big earthquake damaged buildings in 849 AD – many people moved across the river, or left town.

But when a great earthquake knocked down most of Lisbon in Portugal in 1755, Portugal again used the opportunity to rebuild a more modern city (and Voltaire wrote about it in his Candide).

Today, while we haven’t figured out how to stop earthquakes, we do know how to cause them. Fracking for oil is causing many small earthquakes, and it may cause a large one someday soon if we don’t stop.

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